A conventional image reading apparatus for reading a document image by using a linear image sensor has an arrangement shown in FIG. 10. In FIG. 10, reference numeral 101 denotes a document illumination lamp; 102 to 104, first to third mirrors; 105, a lens; 106, a linear image sensor; 107, a document glass table; 108, a document sheet; and 109, a white plate serving as a shading correction reference. In the image reading apparatus of FIG. 10, the first to third mirrors 102 to 104 and the document illumination lamp 101 move in directions indicated by arrows in FIG. 10 to perform optical scanning (subscanning), thereby two-dimensionally reading the document sheet 108.
FIG. 11 shows an arrangement when the image reading apparatus shown in FIG. 10 is equipped with an automatic document feeder. The automatic document feeder feeds a plurality of document sheets 108 on a stack tray 201 one by one to the document glass table 107 of the image reading apparatus. The document sheet 108 is conveyed by circularly driving an automatic document feeding belt 202 by driving rollers 203. For descriptive convenience, FIGS. 10 and 11 show two document illumination lamps 101, two first mirrors 102, two second mirrors 103, and two third mirrors 104. In practice, one document illumination lamp 101, one first mirror 102, one second mirror 103, and one third mirror 104 are arranged.
One of image reading modes in which a document image is read by using the image reading apparatus and automatic document feeder is a so-called “flow scanning” mode in which the document sheet 108 moving at a predetermined speed on the document glass table 107 is read without moving the optical system. In the “flow scanning mode”, the image reading apparatus moves the first to third mirrors 102 to 104 so as to read a predetermined position of the document glass table 107. In this state (mirror positions are fixed after the mirrors are moved), the automatic document feeder moves the document sheet 108 at a predetermined speed to read the document sheet 108. In this case, the position on the document glass table 107 where the image reading apparatus reads the document sheet 108 is called a “flow scanning position”, and is a fixed position as represented by the broken line in FIG. 12. A position Δ in FIG. 12 is a document abutment position. The document sheet 108 is conveyed by the automatic document feeder in a direction indicated by the arrow in FIG. 12.
In the “flow scanning mode” in which a document sheet is read by using the automatic document feeder, the document sheet is merely moved in one direction. This mode has an advantage in that the convey time interval between document sheets sequentially conveyed to the flow scanning position is shorter than that of optical scanning operation when the image reading apparatus successively reads a large number of document sheets.
However, the prior art suffers the following problems. If dust, dirt, or the like is attached to the flow scanning position on the document glass table 107, as shown in FIG. 12, in reading a document sheet in the “flow scanning mode” by the conventional image reading apparatus, a black streak 303 is generated at a corresponding main scanning position of a read image 302 obtained by reading a document image 301 on the document sheet 108 by the linear image sensor. In the prior art, generation of a black streak by dust, dirt, or the like attached to the flow scanning position on the document glass table 107 can only be prevented by cleaning the surface of the document glass table.